The invention relates to a novel system for the secret transmission of audio signals, and particularly audio signals associated with television pictures.
The system uses an encoding arrangement, at least one transmission channel and a decoding arrangement. The encoding arrangement comprise in a cascade arrangement coupled to an input terminal:
a first filter,
a first modulator connected to a first fixed-frequency modulation signal generator,
a second filter,
a second modulator connected via a first change-over switch to two outputs of a first generator supplying at these two outputs two modulation signals having two different frequencies, said change-over switch being controlled by a first pseudo-random generator for selecting one or the other of these two modulation signals, and
a third filter.
The decoding arrangement comprises a field sync pulse generator, a separator for separating a reference signals received simultaneously with the encoded audio signals, and, in a cascade arrangement coupled to an input terminal:
a fourth filter,
a third modulator connected via a second change-over switch to two outputs of a second generator supplying at these two outputs two modulation signals having two different frequencies, said change-over switch being controlled by a second pseudo-random generator for selecting one or the other of these two modulation signals,
a fifth filter,
a fourth modulator connected to a second fixed frequency modulation signal generator,
a sixth filter.